Internal combustion engines emit undesirable pollutants in their exhaust stream. One such pollutant is nitrogen oxides, comprising nitrogen monoxide and nitrogen dioxide (hereinafter referred to simply as “NOx”). NOx is generated from automobile engines such as diesel engines, and other combustion devices. Accordingly, exhaust system devices are coupled to the engine to limit and/or remove the pollutants from the exhaust system. Technologies have been and continue to be developed to attenuate these emissions.
NOx may be cleaned from exhaust gases of internal combustion engines through the use of catalysis. In addition to removing NOx, other catalysts may be employed to also remove unburned hydrocarbons (HC) and carbon monoxide (CO). When the engine is operated with a lean air/fuel ratio, the catalyst is efficient at removing the HCs and COs because of the extra oxygen in the exhaust gas. However, the extra oxygen tends to inhibit the removal of NOx.
In the case of exhaust gas from gasoline engines, NOx is usually removed by using so-called three-way catalysts. Also, in the case of large, stationary combustion apparatus, such as internal combustion engines for cogenerators, metal oxide catalysts such as V2O5 are used, and ammonia is introduced into the exhaust gas stream, whereby nitrogen oxides in the exhaust gas are catalytically and selectively reduced.
Conventional Selective Catalytic Reduction (SCR) of NOx involves injection of a typically aqueous urea solution or reductant into the exhaust system ahead of the SCR catalyst. Common reductants include aqueous urea in conjunction with selective catalytic reduction, and perhaps hydrocarbon diesel fuel, for the supplemental heat necessary to initiate particulate trap regeneration. As used herein the term “urea” is meant to encompass urea in all of its commercial forms, including those containing: ammelide; ammeline; ammonium carbonate; ammonium bicarbonate; ammonium carbamate; ammonium cyanate; ammonium salts of inorganic acids, including sulfuric acid and phosphoric acid; ammonium salts of organic acids, including formic and acetic acid; biuret; cyanuric acid; isocyanic acid; melamine and tricyanourea.
The reductant dosing system is required to accurately meter the reductant into the exhaust system, while being robust in service and responsive to the engine or after treatment control system. Accordingly, most prior art dosing systems have been designed for delivery of reductant in a two fluid system, namely the reductant and a supply of pressurized air. Such systems make use of a simple low pressure metering pump for delivering the reductant into a mixing chamber where it co-mingles with pressurized air from an onboard source. This mixture is conducted through a pipe to the remote dosing location in the exhaust where it exits through a simple atomizing nozzle into the exhaust stream. A typical air atomizing pressure for this type of system might be 2 bar. By way of example, EP 1 149 991 describes a two fluid system of the aforementioned type.
Since urea is not essential to the functioning of the engine, it is possible to operate the engine even when the supply of urea is low or has been depleted altogether. However, such a condition is unacceptable, not least because the engine then no longer complies with statutory regulations for emissions levels. It is therefore known to provide the urea tank with a urea level sensor (not shown) so that imminent exhaustion of the supply can be forewarned. U.S. Pat. No. 6,063,350, for example, describes a reagent dosing system in which such a level sensor is used for this purpose.
Aqueous urea is a common reagent in such systems, but the characteristics of the fluid are problematic in certain respects. For example, aqueous urea can form hard salt crystals which clog mechanisms and disrupt regular operation. For this reason, fluid level sensors of the mechanical type, which employ a float on a pivoting arm connected to a variable resistance coil, are unsuited for use in a urea supply tank. Typically, therefore, a more expensive urea level sensor is employed that is immune from clogging.
It is one object of the invention to provide a system for use in delivering reagent in an SCR exhaust gas cleaning system of an internal combustion engine in which the requirement for a separate reagent level sensor is removed altogether.